Dynamic coupling of excitatory and inhibitory responses in the medial nucleus of the trapezoid body

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The neuronal representation of acoustic amplitude modulations is an important prerequisite for understanding the processing of natural sounds. We investigated this representation in the medial nucleus of the trapezoid body (MNTB) of the Mongolian gerbil using sinusoidal amplitude modulations (SAM). Depending on the SAM's carrier frequency (fC) MNTB cells either increase or decrease their discharge rates, indicating underlying excitatory and inhibitory/suppressive mechanisms. As natural sounds typically are composed of multiple spectral components we investigated how stimuli containing two spectral components are represented in the MNTB, especially when they have opposing effects on the discharge rate. Three conditions were compared: SAM stimuli (1) with rate-increasing fC, (2) with rate-increasing fC and an additional unmodulated rate-decreasing pure tone, and (3) with rate-decreasing fC and an unmodulated, rate-increasing pure tone. We found that responses under all three conditions showed comparable strength of phase-locking. Adding a rate-decreasing tone to a rate-increasing SAM increased phase-locking for modulation frequencies (fAM) of ≤ 600 Hz. A comparison of two possible coding strategies – phase-locking vs. envelope reproduction – indicates that both strategies are realized to different degrees depending on the fAM. We measured latencies for following modulations in rate-increasing and rate-decreasing SAMs using a modified reverse correlation approach. Although latencies varied between 2.5 and 5 ms between cells, a decrease in rate consistently followed an increase in rate with a delay of about 0.2 ms in each cell. These results suggest a temporally precise representation of rate-increasing and rate-decreasing stimuli at the level of the MNTB during dynamic stimulation.

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